Latent Heat and Clouds

National Standards Addressed

Earth and Space Science, Grades 5-8: Clouds, formed by the condensation of water vapor, affect weather and climate.

Earth and Space Science, Grades 5-8: Water, which covers the majority of the earth’s surface, circulates through the crust, oceans, and atmosphere in what is known as the water cycle. Water evaporates from the earth’s surface, rises and cools as it moves to higher elevations, condenses as rain or snow, and falls to the surface where it collects in lakes, oceans, soil, and in rocks underground.

Earth and Space Science, Grades 5-8: The atmosphere is a mixture of nitrogen, oxygen, and trace gases that include water vapor. The atmosphere has different properties at different elevations.

Earth and Space Science, Grades 9-12: Movement of matter between reservoirs is driven by the earth’s internal energy and external sources of energy…

Physical Science, Grades K-4: Heat can be produced in many ways, such as burning, rubbing or mixing one substance with another. Heat can move from one object to another by conduction.

Physical Science, Grades 5-8: Energy is a property of many substances and is associated with heat, light, electricity, mechanical motion, sound, nuclei, and the nature of a chemical. Energy is transferred in many ways.

Quick Summary of Lesson

This activity has students explore latent heat and its relation to clouds in the atmosphere.

6. Have high school students answer questions on Student Activity Sheet.

Student Activity Sheet

Please click here for student activity sheets. All activities on the Windows to the Universe site may be printed and reproduced if being used for educational purposes.

Notes to the Teacher

The reheater pack can be bought at Arbor Scientific. Here's the info from their site:
"Dramatically demonstrate crystallization and the release of latent heat! This fascinating hands-on experiment utilizes a patented 3 x 6" pouch of supercooled sodium acetate to visually demonstrate crystallization and the subsequent release of latent heat. Indefinitely repeatable, simply boil and cool the ReHeater® pouch to return its contents to the supercooled state. Best of all, you can use it to introduce conservation of energy to students of all ages. The instruction manual covers history, practical applications, and suggestions for qualitative and quantitative experiments."
$8 at Arbor Scientific

What is happening in this activity is that the liquid sodium acetate crystallizes releasing latent heat. When a liquid goes to a solid, energy in the form of latent heat is released. Of course, this release is "invisible" to us, but can be "seen" when the water temperature in our deli containers rises.

From middle school students, I just collect individual data charts and graphs at the end of the activity. You may choose to have high school students answers the discussion questions on the student activity sheet. Several of the questions are based on a great (and thin) book called The Stories Clouds Tell by Margaret A. LeMone. This book was put out by Project Atmosphere, the American Meteorological Society.

Here are answers to the questions on the Student Activity Sheet. Questions 1-3 depend on results from the activity and so answers will vary. The reaction in the heater pack is exothermic. As water vapor condenses into cloud droplets, heat energy in the water droplet is released. The mass of the cumulonimbus cloud is 1,000,000,000 kg (10^9 kg), 2.2 x 10^9 pounds or 1.1 x 10^6 tons. If 10^9 kg of water falls on 10^8 square meteres, there is 10 kg per square meter or 10000 grams/(100cm x100cm) or 1gm/cm^2. (10 km x 1000 m)/km divided by 5 m/s gives 2000 seconds or about 33 minutes for the air to get to from the bottom of the cloud to the top. This is about how long small, weak thunderstorms actually last.

Water vapor in the atmosphere is extremely important. It is water vapor that changes into both liquid and solid cloud particles that fall to the Earth as precipitation. When water vapor changes to liquid water or ice, it releases large amounts of latent heat. Latent heat is a huge source of atmospheric energy that is needed for storms, thunderstorms or hurricanes to form. For each kilogram of cloud particles that form, over 120,000 calories are given off. When water vapor evaporates from the ocean or a lake or a river, that water vapor may be transported far from the original point of evaporation. It might be transported from warmer, tropical areas all the way to polar regions. At this point, the water vapor would be cooled and would condense, releasing latent heat of condensation. So what started out as evaporation of water many miles south, could end up prompting a snowstorm in Canada! So we see that understanding water vapor and latent heat release are vital to understanding atmospheric processes and weather systems.